Recreations in Astronomy eBook

When the total eclipse of the sun occurred in 1878,
[Page 138] astronomers were determined that the question
of the existence of an intra-mercurial planet should
be settled. Maps of all the stars in the region
of the sun were carefully studied, sections of the
sky about the sun were assigned to different observers,
who should attend to nothing but to look for a possible
planet. It is now conceded that Professor Watson,
of Ann Arbor, actually saw the sought-for body.

Mercury shines with a white light nearly as bright
as Sirius; is always near the horizon. When nearly
between us and the sun, as at D (Fig. 46, p. 113),
its illuminated side nearly opposite to us, we, looking
from E, see only a thin crescent of its light.
When it is at its greatest angular distance from the
sun, as A or C, we see it illuminated like the half-moon.
When it is beyond the sun, as at E, we see its whole
illuminated face like the full-moon.

The variation of its apparent size from the varying
distance is very striking. At its extreme distance
from the earth it subtends an angle of only five seconds;
nearest to us, an angle of twelve seconds. Its
distance from the earth varies nearly as one to three,
and its apparent size in the inverse ratio.

[Page 139] When Mercury comes between the earth and
the sun, near the line where the planes of their orbits
cut each other by reason of their inclination, the
dark body of Mercury will be seen on the bright surface
of the sun. This is called a transit. If
it goes across the centre of the sun it may consume
eight hours. It goes 100,000 miles an hour, and
has 860,000 miles of disk to cross. The transit
of 1818 occupied seven and a half hours. The
transits for the remainder of the century will occur: